libc: wunlock() part 2

the initial issue was that wunlock() would wakeup readers while
holding the spinlock causing deadlock in libthread programs where
rendezvous() would do a thread switch within the same process
which then can acquire the RWLock again.

the first fix tried to prevent holding the spinlock, waking up
one reader at a time with releasing an re-acquiering the spinlock.
this violates the invariant that readers can only wakup writers
in runlock() when multiple readers where queued at the time of
wunlock(). at the first wakeup, q->head != nil so runlock() would
find a reader queued on runlock() when it expected a writer.

this (hopefully last) fix unlinks *all* the reader QLp's atomically
and in order while holding the spinlock and then traverses the
dequeued chain of QLp structures again to call rendezvous() so
the invariant described above holds.

0 files changed, 0 insertions, 0 deletions